1. Field of the Invention
This invention relates to a vibrometer, and more particularly, is suitable for a vibrometer used by hand-holding on the vibrating surface of object to be measured, out of vibrometers in which the vibrating quantity of the operating plant is measured for security and failure check. The vibrometer of this invention is also suitable for accelerometers out of vibrometers containing an accelerometer, a speedometer and a displacement meter. Therefore, the accelerometer (vibrometer) will be mainly described below.
2. Description of the Related Art
Heretofore, the vibrometer is classified in accordance with the direction of measurement into a uniaxial vibrometer having one axis of measurement, and a three-axial vibrometer having axes of measurement in three directions, etc.
Besides, in accordance with the method of use, the vibrometer is classified into a fixed type of the vibrometer which is screwed or bonded on the vibrating surface of the object to be measured and a hand-hold type of the vibrometer which is hand-held by operator on the vibrating surface of the object to be measured.
In these vibrometers generally, the measurement area which can be used the vibrometer is mostly determined on the basis of the spring constant (hereinafter, referred to as "contact spring constant") and the mass of a vibration sensor, the contact spring constant existing between the contact surface (or the hand-held portion) facing on the surface the vibration of which is measured of the vibrometer and the vibration measurement surface.
Specifically, the relation (hereinafter, referred to as "frequency characteristic") between the vibration on the surface the vibration of which is measured and the sensitivity of the vibration sensor is shown as a graph in FIG. 1 when sensitivity is represented in the axis of ordinates and the frequency of the surface the vibration of which is measured is represented in the axis of abscissas. In measuring vibration, the frequency area Fie, which is generated according to the contact spring constant and the mass of vibration sensor and which is approximately less than 1/3 of the resonance frequency f1 (hereinafter, referred to as "contact resonance frequency) between the vibrometer and the surface the vibration of which is measured, is generally used as a measurement data.
In this case, if the vibration sensors 1 and 2 shown in FIGS. 2A and 2B are equalized regarding respective mass, sensitivity, and the contact area, comparing the frequency characteristic, in the case that the vibrating direction is parallel to the surface 3 the vibration of which is measured as shown by the arrow "a" in FIG. 2A and the axis of measurement K1 of the vibration sensor 1 set on the surface 3 is parallel to the surface 3, and the frequency characteristic, in the case that the vibrating direction is perpendicular to the surface 3 the vibration of which is measured as shown by the arrow "b" in FIG. 2B and the axis of measurement K2 of the vibration sensor 2 set on said surface 3 is perpendicular to the surface 3, as shown in FIG. 3, the contact resonance frequency fa in the case of FIG. 2A becomes lower than the contact resonance frequency fb in the case of FIG. 2B.
As described above, because the contact spring constant between the vibration sensor 1 and the surface 3 the vibration of which is measured and the contact spring constant between the vibration sensor 2 and the surface 3 differ in accordance with the vibrating directions, the difference between the contact resonance frequencies fa and fb occurs depending on the direction of the surface 3 the vibration of which is measured, and the direction of the axis of measurement K1 or K2 of the vibration sensor 1 or 2 set on said surface 3. In the vibration sensor, the contact spring constant, in the case that the axis of measurement is perpendicular to the surface 3 and the vibrating direction of the object the vibration of which is measured is perpendicular to the surface 3, is generally larger than the contact spring constant, in the case that the axis of measurement is parallel to the surface 3 and the vibrating direction of the object the vibration of which is being measured is parallel to the surface 3.
Further, in the vibrometer, the fixed type of the vibrometer is fixed on the object the vibration of which is measured for the surface. On the contrary, the hand-hold type of vibrometer is formed into spherical shape of 3 to 10 [mm] in the end of probe so as to contact with the surface the vibration of which is measured nearly by the dot contact, and the hand-held force is usually used on the order of 1 to 2 [Kgf]. Therefore, the hand-held type of vibrometer has a problem that the contact spring constant is low since the area contacted on the surface the vibration of which is measured is smaller than that of the fixed type, so that the contact resonance frequency is small.
However, the hand-held type of vibrometer can easily measure the quantity of vibration only by hand-holding the vibrometer on the surface the vibration of which is measured. Thus, for example, in the case when a lot of points to be measured are measured continuously, the hand-held type of vibrometer can perform easier and more effective measurement than the fixed type. Therefore, the hand-held type of vibrometer is suitable for the case where the quantity of vibration of the operating plant is measured for the purpose of security and failure check.
Heretofore, in such measurement, the quantity of vibration of the direction which is perpendicular to the surface of the object the vibration of which is measured (hereinafter, this direction is called "Z-axis direction"), and the quantity of vibration of two directions which are parallel to said surface and are perpendicular to each other (hereinafter, these directions are called "X-axis direction" and "Y-axis direction" respectively), are measured, and then the vibration state of the object the vibration of which is measured has been analyzed three-dimensionally based on the measured result.
As the method for detecting in this case, generally, there is a method that three uniaxial vibrometers are fixed or hand-held on one point on the surface which orthogonalize with X-axis direction, one point on the surface which orthogonalize with Y-axis direction and X-axis direction.
However, the detection efficiency is low in this detecting method, and moreover the points the vibration of which is measured in the directions of respective axes are not same, so that strictly speaking, this is not the measurement of vibration in desired one point. Therefore, as a whole, there is a problem that the measured result of the object the vibration of which is measured is not obtained accurately.
As a means for solving this problem, heretofore, the fixed type of a three-axial vibrometer 10 having the construction shown in FIG. 4 has been proposed.
That is, the three-axial vibrometer 10 is composed of the vibration sensors 12A, 12B and 12C. The vibration sensors 12A, 12B and 12C are equipped on the side surfaces 11A, 11B and 11C which are next to each other of the vibration sensor mount block 11 formed to the rectangular parallelepipeds shape in order that the respective axes of measurement K10, K11 and K12 become perpendicular to the side surfaces 11A, 11B and 11C. The opposite side surface 11D of the surface 11A on which the vibration sensor 12A is equipped for measuring the vibration in Z-axis direction is fixed on the surface of the object the vibration of which is measured (not shown) with a bond or screws, so that the vibration in three axes directions, which are X-, Y- and Z-axis directions, can be detected at one point simultaneously.
However, in this type of the vibrometer 10, if the vibrometer is fixed on the surface the vibration of which is measured with screws, etc., since the contact spring constant is large with respect to the vibration in the direction (Z-axis direction) which is perpendicular to the surface the vibration of which is measured, the contact resonance frequency is high. Therefore, the frequency area in Z-axis direction which can be measured by the vibration sensor 12A expands. On the contrary, the frequency area in X-axis and Y-axis directions which can be measured by the vibration sensors 12B and 12C becomes narrow, since the contact spring constant is small with respect to the vibration in the directions (X-axis and Y-axis directions) which are parallel to the surface the vibration of which is measured, the contact resonance frequency becomes low in comparison with the contact spring constant in Z-axis direction. Therefore, the frequency area in X-axis and Y-axis directions which can be measured by the vibration sensors 12B and 12C becomes narrow. Hence, in this type of the vibrometer 10, there is a problem that the frequency area in the X-axis and Y-axis directions obtained as a measured result becomes only about 1/5 of the frequency area in the Z-axis direction.
Moreover, if the conventional fixed type of the three-axis vibrometer 10 is hand-held on the object the vibration of which is measured to measure the vibration, the measurable frequency area in the X-axis and Y-axis directions becomes further narrow, in comparison with the case where the vibrometer 10 is fixed by screws, etc. on the surface the vibration of which is measured.
Furthermore, a vibrometer (not shown) can be conceived in which a probe is set on the conventional three-axis vibrometer 10, and the vibration is measured by hand-holding the end of probe of this vibrometer on the object the vibration which is measured while the vibrometer is hand-held by operator. However, the end of probe is formed into a sphere surface and contacts with the surface the vibration of which is measured in the state of almost dot contact, so that, in comparison with the case where the three-axis vibrometer 10 in FIG. 4. is used by being hand-held, the contact spring constant in X-axis and Y-axis directions become small value. Therefore, the flat frequency characteristics is not obtained till the frequency area of more than 2 [kHz] which is needed in the failure check so that the hand-held type of three-axis vibrometer has not been realized for practical use.